Backing layer of a donor element for adjusting the focus on an imaging laser

ABSTRACT

A process for adjusting the energy of an imaging laser for imaging of a thermally imageable element and thermally imageable elements suitable for this purpose are described. The process comprises the steps of: (a) providing an imaging unit having a non-imaging laser and an imaging laser, the non-imaging laser having a light detector which is in communication with the imaging laser; (b) contacting a receiver element with the thermally imageable element in the imaging unit, the thermally imageable element comprising a thermally imageable layer on a front side of a base element and a light attenuated layer on a back side of the base element comprising a light attenuating agent; (c) actuating the non-imaging laser to expose the thermally imageable element and the receiver element to an amount of light energy sufficient for the light detector to detect the amount of light reflected from the light attenuated layer of the thermally imageable element; and (d) actuating the imaging laser to focus the imaging laser in order to expose the thermally imageable element to an amount of light energy sufficient for imaging the thermally imageable element. The light attenuation is achieved by use of a light attenuating agent or by physically roughening a support.

This application claims benefit of 60/256,242 Dec. 15, 2000

FIELD OF THE INVENTION

This invention relates to processes and products for effectinglaser-induced thermal transfer imaging. More specifically, the inventionrelates to a modified thermally imageable element and its use inadjusting the focus of the imaging laser for imaging thermally imageableelements.

BACKGROUND OF THE INVENTION

Laser-induced thermal transfer processes are well known in applicationssuch as color proofing, electronic circuits, and lithography. Suchlaser-induced processes include, for example, dye sublimation, dyetransfer, melt transfer, and ablative material transfer.

Laser-induced processes use a laserable assemblage comprising (a) athermally imageable element that contains a thermally imageable layer,the exposed areas of which are transferred, and (b) a receiver elementhaving an image receiving layer that is in contact with the thermallyimageable layer. The laserable assemblage is imagewise exposed by alaser, usually an infrared laser, resulting in transfer of exposed areasof the thermally imageable layer from the thermally imageable element tothe receiver element. The (imagewise) exposure takes place only in asmall, selected region of the laserable assemblage at one time, so thattransfer of material from the thermally imageable element to thereceiver element can be built up one pixel at a time. Computer controlproduces transfer with high resolution and at high speed.

The equipment used to image thermally imageable elements is comprised ofan imaging laser, and a non-imaging laser, wherein the non-imaging laserhas a light detector that is in communication with the imaging laser.Since the imaging and non-imaging lasers have emissions at differentwavelengths, problems occur with the focus of the imaging laser.

A need exists for a process for adjusting the focus of the imaging laserfor imaging a thermally imageable element.

SUMMARY OF THE INVENTION

The invention provides a thermal imaging process that uses modifiedthermally imageable elements that allow for the adjusting of the focusof an imaging laser in imaging thermally imageable elements. Theinvention greatly modifies the imaging latitude of the thermallyimageable element by facilitateing laser focus and imaging from color tocolor.

The invention relates to a process for adjusting the focus of an imaginglaser for imaging a thermally imageable element comprising the steps of:

(a) providing an imaging unit having a non-imaging laser and an imaginglaser, the non-imaging laser having a light detector which is incommunication with the imaging laser;

(b) contacting a receiver element with the thermally imageable elementin the imaging unit, the thermally imageable element comprising athermally imageable layer on a front side of a base element and a lightattenuated layer on a back side of the base element comprising a lightattenuating agent;

(c) actuating the non-imaging laser to expose the thermally imageableelement and the receiver element to an amount of light energy sufficientfor the light detector to detect the amount of light reflected from thelight attenuated layer of the thermally imageable element and thereceiver element; and

(d) actuating the imaging laser to focus the imaging laser in order toexpose the thermally imageable element to an amount of light energysufficient for imaging the thermally imageable element, the focus oflight energy being determined by the amount of light reflected from thelight attenuated layer of the thermally imageable element andcommunicated to the imaging laser by the light detector.

The light attenuating agent may be selected from the group consisting ofan absorber, a diffuser, and mixtures thereof.

The process may further comprises the steps of:

(a) imaging the thermally imageable element to form imaged andnon-imaged areas; and

(b) separating the imaged thermally imageable element from the receiverelement to form an image on the receiver element.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a thermally imageable element (10) useful in theinvention having a support (11); a base element having a coatablesurface comprising an optional ejection layer or subbing layer (12) anoptional heating layer (13); a thermally imageable colorant-containinglayer (14) and a back coating (15) on the back surface of the support(11) comprising a light attenuating agent.

FIG. 2 illustrates a receiver element (20), optionally having aroughened surface, useful in the invention having a receiver support(21) and a image receiving layer (22).

FIGS. 3 and 4 illustrate the positioning of the thermally imageableelement having a light attenuated layer (10), the receiver element (20),and the optional carrier element 71 on drum (70) prior to vacuumdrawdown and laser imaging.

FIG. 5 illustrates a non-imaging autofocus probe beam light as it isreflected from the thermally imageable element, the receiver element andthe carrier element (71) wherein the thermally imageable element doesnot contain a light attenuated layer.

DETAILED DESCRIPTION OF THE INVENTION

Processes and products for laser induced thermal transfer imaging aredisclosed wherein thermally imageable elements providing modifiedimaging characteristics are provided.

Before the processes of this invention are described in further detail,several different exemplary laserable assemblages made up of thecombination of a receiver element and a thermally imageable element willbe described. The processes of this invention are fast and are typicallyconducted using one of these exemplary laserable assemblages.

Imageable Element

As shown in FIG. 1, an exemplary thermally imageable element useful forthermal imaging in accordance with the processes of this inventioncomprises a thermally imageable layer, which typically in a proofingapplication is a colorant-containing layer (14) and a base elementhaving a coatable surface which comprises an optional ejection layer orsubbing layer (12) and optionally a heating layer (13). Each of theselayers has separate and distinct functions. Optionally, a support forthe thermally imageable element (11) may also be present. The thermallyimageable element comprises a light attenuating layer (15) whichcomprises a light attenuating agent. In one embodiment, the optionalheating layer (13) may be present directly on the support (11).Alternately, an overcoat layer may also be provided on the thermallyimageable colorant-containing layer (14).

The thermally imageable element may be simply a laser imageable elementfor a laser imaging process capable of imaging an imageable element asdescribed herein by non-thermal methods.

The light attenuating agent may be selected from the group consisting ofan absorber, a diffuser, and mixtures thereof. Depending on the range atwhich the non-imaging laser operates, such as about 300 nm to about 1500nm, the absorbers and diffusers should be selected to operate in thesame range. Depending on the wavelength range at which the imaging laseroperates,which can be from about 300 nm to about 1500 nm, the absorbersand diffusers can be inoperable in the same range. For example, if thenon-imaging laser operates in about the 670 nm region and the imaginglaser at 830 nm, it is preferred that the absorbers and diffusersoperate to absorb or diffuse light in the 670 nm region and the abilityof these materials to absorb or diffuse light at 830 nm can be poor.Some examples of light absorbers include any blue phthalocyaninepigments with significant absorption in about the 670 nm range andminimal absoption at 830 nm; such as C.I. Pigment Blue 15 or 15-3, anduniversally absorbing black pigments such as any carbon black. Someexamples of light diffusers are materials which scatter light or scatterand absorb light. They can include white pigments such as titaniumdioxide, or combinations (extensions) of white pigments such as:titanium dioxide, barium sulfate, calcium carbonate, oxides, sulfates,carbonates of silicon (i.e. silicon dioxide) and magnesium, etc.Commercial examples of white pigments would include DuPont's TiPure®grades of titanium dioxide. Carbon black examples include any Monarch®,Regal®, Elftex® or Sterling® carbon blacks from Cabot Corporation,Boston, Mass. Blue pigment examples would be the Sunfast® bluephthalocyanine pigment 15-3 series from Sun Chemical Corporation,Cincinnati, Ohio.

The use of dyes or combinations of dyes could also conceivably beemployed to affect the imaging properties of the herein describedthermal imaging system. To one skilled in the art, combinations of blue,red and green dyes could be substituted for pigments. However, adisadvantage in using dyes is the lack of light fastness and tendency tomigrate out of the layer.

The light attenuated layer can be applied by known coating techniques.The coatable composition can comprise a dispersion of the lightattenuating agent in a binder. A suitable binder can be polymeric andcan be the same as the polymers employed in the thermally imageablelayer. A minor amount of a surfactant can also be employed. Typically,the binder is a copolymer of methylmethacrylate and n-butylmethacrylateand the surfactant is a fluoropolymer. Usually, the components of thelight attenuated layer are mixed into an an aqueous dispersion which isapplied as a coating to the back side of the base element byconventional techniques and dried.

The amount of the light attenuating agent is combined with the binderand other components of the back coating in an amount effective toabsorb or diffuse the light from the non imaging laser. The lightattenuated layer can comprise a polymeric binder which can be the sameas that used in the thermally imageable layer. The light attenuatingagent is used in the light attenuated layer in an amount sufficient toachieve an absorbance ranging from about 0.1 to about 2.0, typicallyfrom about 0.3 to about 0.9 even more typically about 0.6. Theabsorbance is a dimensionless figure which is well known in the art ofspectroscopy. At an absorbance above about 2.0 the base is likely to betoo highly absorbing for the imaging process and below about 0.1 theremight not be a sufficient attenuating effect.

Base Element:

Typically, the base element (12) is a thick (400 gauge) coextrudedpolyethylene terephthalate film. Alternately, the base element may bepolyester film, specifically polyethylene terephthalate that has beenplasma treated to accept the heating layer such a the Melinex® line ofpolyester films made by DuPontTeijinFilms™ a joint venture of DuPont andTeijin Limited. When the base element is plasma treated, a subbing layeror ejection layer is usually not provided on the support. Backing layersmay optionally be provided on the support. These backing layers maycontain fillers to provide a roughened surface on the back side of thebase element, i.e. the side opposite from the base element (12).Alternatively, the base element itself may contain fillers, such assilica, to provide a roughened surface on the back surface of the baseelement. Alternately, the base element may be physically roughened toprovide a roughened surface on one or both surfaces of the base elementsaid roughening being sufficient to scatter the light emitted from thenon-imaging laser. Some examples of physical roughening methods includesandblasting, impacting with a metal brush, etc. If a support isemployed it may be the same or different from the base element.Typically, the support is a thick polyethylene terephthalate film.

Ejection or Subbing Layer:

The optional ejection layer, which is usually flexible, or optionalsubbing layer, which may be on one side of the base element (12), asshown in FIG. 1, is the layer that provides the force to effect transferof the thermally imageable colorant-containing layer to the receiverelement in the exposed areas. When heated, this layer decomposes intogaseous molecules providing the necessary pressure to propel or ejectthe exposed areas of the thermally imageable colorant-containing layeronto the receiver element. This is accomplished by using a polymerhaving a relatively low decomposition temperature (less than about 350°C., typically less than about 325° C., and more typically less thanabout 280° C.). In the case of polymers having more than onedecomposition temperature, the first decomposition temperature should belower than 350° C. Furthermore, in order for the ejection layer to havesuitably high flexibility and conformability, it should have a tensilemodulus that is less than or equal to about 2.5 Gigapascals (GPa),specifically less than about 1.5 GPa, and more specifically less thanabout 1 Gigapascal (GPa). The polymer chosen should also be one that isdimensionally stable. If the laserable assemblage is imaged through theejection layer, the ejection layer should be capable of transmitting thelaser radiation, and not be adversely affected by this radiation.

Examples of suitable polymers for the ejection layer include (a)polycarbonates having low decomposition temperatures (Td), such aspolypropylene carbonate; (b) substituted styrene polymers having lowdecomposition temperatures, such as poly(alpha-methylstyrene); (c)polyacrylate and polymethacrylate esters, such as polymethylmethacrylateand polybutylmethacrylate; (d) cellulosic materials having lowdecomposition temperatures (Td), such as cellulose acetate butyrate andnitrocellulose; and (e) other polymers such as polyvinyl chloride;poly(chlorovinyl chloride) polyacetals; polyvinylidene chloride;polyurethanes with low Td; polyesters; polyorthoesters; acrylonitrileand substituted acrylonitrile polymers; maleic acid resins; andcopolymers of the above. Mixtures of polymers can also be used.Additional examples of polymers having low decomposition temperaturescan be found in U.S. Pat. No. 5,156,938. These include polymers whichundergo acid-catalyzed decomposition. For these polymers, it isfrequently desirable to include one or more hydrogen donors with thepolymer.

Specific examples of polymers for the ejection layer are polyacrylateand polymethacrylate esters, low Td polycarbonates, nitrocellulose,poly(vinyl chloride) (PVC), and chlorinated poly(vinyl chloride) (CPVC).Most specifically are poly(vinyl chloride) and chlorinated poly(vinylchloride).

Other materials can be present as additives in the ejection layer aslong as they do not interfere with the essential function of the layer.Examples of such additives include coating aids, flow additives, slipagents, antihalation agents, plasticizers, antistatic agents,surfactants, and others which are known to be used in the formulation ofcoatings.

Alternately, a subbing layer may optionally be applied onto the baseelement (12) in place of the ejection layer resulting in a thermallyimageable element having in order at least one subbing layer on one sideof the base element (12), at least one heating layer (13), and at leastone thermally imageable colorant-containing layer (14). Some suitablesubbing layers include polyurethanes, polyvinyl chloride, cellulosicmaterials, acrylate or methacrylate homopolymers and copolymers, andmixtures thereof. Other custom made decomposable polymers may also beuseful in the subbing layer. Specifically useful as subbing layers forpolyester, specifically polyethylene terephthalate, are acrylic subbinglayers. The subbing layer may have a thickness of about 100 to about1000 A.

Heating Layer

The optional heating layer (13), as shown in FIG. 1, is deposited on theoptional flexible ejection or subbing layer. The function of the heatinglayer is to absorb the laser radiation and convert the radiation intoheat. Materials suitable for the layer can be inorganic or organic andcan inherently absorb the laser radiation or include additionallaser-radiation absorbing compounds.

Examples of suitable inorganic materials are transition metal elementsand metallic elements of Groups IIIA, IVA, VA, VIA, VIIIA, IIB, IIIB,and VB of the Period Table of the Elements (Sargent-Welch ScientificCompany (1979)), their alloys with each other, and their alloys with theelements of Groups IA and IIA. Tungsten (W) is an example of a Group VIAmetal that is suitable and which can be utilized. Carbon (a Group IVBnonmetallic element) can also be used. Specific metals include Al, Cr,Sb, Ti, Bi, Zr, Ni, In, Zn, and their alloys and oxides. TiO₂ may beemployed as the heating layer material.

The thickness of the heating layer is generally about 10 Angstroms toabout 0.1 micrometer, more specifically about 20 to about 60 Angstroms.

Although it is typical to have a single heating layer, it is alsopossible to have more than one heating layer, and the different layerscan have the same or different compositions, as long as they allfunction as described above. The total thickness of all the heatinglayers should be in the range given above.

The optical density of the heating layer at the wavelength of thenon-imaging laser is typically in the order of greater than about 0.1and less than about 1.0 transmission density.

The heating layer(s) can be applied using any of the well-knowntechniques for providing thin metal layers, such as sputtering, chemicalvapor deposition, and electron beam.

Thermally Imageable Layer:

The thermally imageable layer, which in a color proofing application istypically a thermally imageable colorant-containing layer (14) is formedby applying a thermally imageable composition, typically containing acolorant, to a base element. For other applications, such as electroniccircuit applications, the thermally imageable layer may not contain acolorant. For these applications, the thermally imageable element maycontain electronically active conductors, insulators, semiconductors, orprecursors to these functions.

When the thermally imageable layer is a colorant-containing layer itcomprises (i) a polymeric binder which is different from the polymer inthe ejection layer, and (ii) a colorant comprising a dye or a pigmentdispersion.

The binder for the colorant-containing layer is a polymeric materialhaving a decomposition temperature that is greater than about 250° C.and specifically greater than about 350° C. The binder should be filmforming and coatable from solution or from a dispersion. Binders havingmelting points less than about 250° C. or plasticized to such an extentthat the glass transition temperature is less than about 70° C. aretypical. However, heat-fusible binders, such as waxes should be avoidedas the sole binder since such binders may not be as durable, althoughthey are useful as cobinders in decreasing the melting point of the toplayer.

It is typical that the polymer of the binder does not self-oxidize,decompose or degrade at the temperature achieved during the laserexposure so that the exposed areas of the thermally imageable layercomprising colorant and binder, are transferred intact for improveddurability. Examples of suitable binders include copolymers of styreneand (meth)acrylate esters, such as styrene/methyl-methacrylate;copolymers of styrene and olefin monomers, such asstyrene/ethylene/butylene; copolymers of styrene and acrylonitrile;fluoropolymers; copolymers of (meth)acrylate esters with ethylene andcarbon monoxide; polycarbonates having higher decompositiontemperatures; (meth)acrylate homopolymers and copolymers; polysulfones;polyurethanes; polyesters. The monomers for the above polymers can besubstituted or unsubstituted. Mixtures of polymers can also be used.

Specific polymers for the binder of the colorant-containing layerinclude, but are not limited to, acrylate homopolymers and copolymers,methacrylate homopolymers and copolymers, (meth)acrylate blockcopolymers, and (meth)acrylate copolymers containing other comonomertypes, such as styrene.

The binder polymer generally can be used in a concentration of about 15to about 50% by weight, based on the total weight of thecolorant-containing layer, specifically about 30 to about 40% by weight.

The colorant of the thermally imageable layer may be an image formingpigment which is organic or inorganic. Examples of suitable inorganicpigments include carbon black and graphite. Examples of suitable organicpigments include color pigments such as Rubine F6B (C.I. No. Pigment184); Cromophthal® Yellow 3G (C.I. No. Pigment Yellow 93); Hostaperm®Yellow 3G (C.I. No. Pigment Yellow 154); Monastral® Violet R(C.I. No.Pigment Violet 19); 2,9-dimethylquinacridone (C.I. No. Pigment Red 122);Indofast® Brilliant Scarlet R6300 (C.I. No. Pigment Red 123); QuindoMagenta RV 6803; Monastral® Blue G (C.I. No. Pigment Blue 15);Monastral® Blue BT 383D (C.I. No. Pigment Blue 15); Monastral® Blue G BT284D (C.I. No. Pigment Blue 15); and Monastral® Green GT 751D (C.I. No.Pigment Green 7). Combinations of pigments and/or dyes can also be used.For color filter array applications, high transparency pigments (that isat least about 80% of light transmits through the pigment) are typical,having small particle size (that is about 100 nanometers).

In accordance with principles well known to those skilled in the art,the concentration of pigment will be chosen to achieve the opticaldensity desired in the final image. The amount of pigment will depend onthe thickness of the active coating and the absorption of the colorant.Optical densities greater than 0.8 at the wavelength of maximumabsorption are typically required. Even higher densities are typical.Optical densities in the 2-3 range or higher are achievable withapplication of this invention.

The optical density of the pigmented layer at the wavelength of thenon-imaging laser may be in the range from greater than about 0.01 toless than about 5.0 transmission density, more typically in the order ofabout 0.2 to about 3.0 transmission density. This density may not becontrolled in selection of the colorants, but the non-imaging laser mustbe able to accommodate at least this range of optical properties.

A dispersant is usually used in combination with the pigment in order toachieve maximum color strength, transparency and gloss. The dispersantis generally an organic polymeric compound and is used to separate thefine pigment particles and avoid flocculation and agglomeration of theparticles. A wide range of dispersants is commercially available. Adispersant will be selected according to the characteristics of thepigment surface and other components in the composition as known bythose skilled in the art. However, one class of dispersant suitable forpracticing the invention is that of the AB dispersants. The A segment ofthe dispersant adsorbs onto the surface of the pigment. The B segmentextends into the solvent into which the pigment is dispersed. The Bsegment provides a barrier between pigment particles to counteract theattractive forces of the particles, and thus to prevent agglomeration.The B segment should have good compatibility with the solvent used. TheAB dispersants of utility are generally described in U.S. Pat. No.5,085,698. Conventional pigment dispersing techniques, such as ballmilling, sand milling, etc., can be employed.

The pigment is present in an amount of from about 15 to about 95% byweight, typically about 35 to about 65% by weight, based on the totalweight of the composition of the colorant-containing layer.

Although the above discussion was directed to color proofing, theelement and process of the invention apply equally to the transfer ofother types of materials in different applications. In general, thescope of the invention is intended to include any application in whichsolid material is to be applied to a receptor in a pattern.

The colorant-containing layer may be coated on the base element from asolution in a suitable solvent, however, it is typical to coat thelayer(s) from a dispersion. Any suitable solvent can be used as acoating solvent, as long as it does not deleteriously affect theproperties of the assemblage, using conventional coating techniques orprinting techniques, for example, gravure printing. A typical solvent iswater. The colorant-containing layer may be applied by a coating processaccomplished using the WaterProof® Color Versatility Coater sold byDuPont, Wilmington, Del. Coating of the colorant-containing layer canthus be achieved shortly before the exposure step. This also allows forthe mixing of various basic colors together to fabricate a wide varietyof colors to match the Pantone® color guide currently used as one of thestandards in the proofing industry.

Thermal Amplification Additive

A thermal amplification additive is typically present in the thermallyimageable colorant-containing layer, but may also be present in theejection layer(s) or subbing layer.

The function of the thermal amplification additive is to amplify theeffect of the heat generated in the heating layer and thus to furtherincrease sensitivity to the laser. This additive should be stable atroom temperature. The additive can be (1) a decomposing compound whichdecomposes when heated, to form gaseous by-products(s), (2) an absorbingdye which absorbs the incident laser radiation, or (3) a compound whichundergoes a thermally induced unimolecular rearrangement which isexothermic. Combinations of these types of additives may also be used.

Decomposing compounds of group (1) include those which decompose to formnitrogen, such as diazo alkyls, diazonium salts, and azido (—N3)compounds; ammonium salts; oxides which decompose to form oxygen;carbonates or peroxides. Specific examples of such compounds are diazocompounds such as 4-diazo-N,N′ diethyl-aniline fluoroborate (DAFB).Mixtures of any of the foregoing compounds can also be used.

An absorbing dye of group (2) is typically one that absorbs in theinfrared region. Examples of suitable near infrared absorbing NIR dyeswhich can be used alone or in combination include poly(substituted)phthalocyanine compounds and metal-containing phthalocyanine compounds;cyanine dyes; squarylium dyes; chalcogenopyryioacrylidene dyes;croconium dyes; metal thiolate dyes; bis(chalcogenopyrylo) polymethinedyes; oxyindolizine dyes; bis(aminoaryl) polymethine dyes; merocyaninedyes; and quinoid dyes. When the absorbing dye is incorporated in theejection or subbing layer, its function is to absorb the incidentradiation and convert this into heat, leading to more efficient heating.It is typical that the dye absorbs in the infrared region. For imagingapplications, it is also typical that the dye has very low absorption inthe visible region.

Absorbing dyes also of group (2) include the infrared absorbingmaterials disclosed in U.S. Pat. Nos. 4,778,128; 4,942,141; 4,948,778;4,950,639; 5,019,549; 4,948,776; 4,948,777 and 4,952,552.

When present in the colorant-containing layer, the thermal amplificationweight percentage is generally at a level of about 0.95-about 11.5%. Thepercentage can range up to about 25% of the total weight percentage inthe colorant-containing layer. These percentages are non-limiting andone of ordinary skill in the art can vary them depending upon theparticular composition of the layer.

The colorant-containing layer generally has a thickness in the range ofabout 0.1 to about 5 micrometers, typically in the range of about 0.1 toabout 1.5 micrometers. Thicknesses greater than about 5 micrometers aregenerally not useful as they require excessive energy in order to beeffectively transferred to the receiver.

Although it is typical to have a single colorant-containing layer, it isalso possible to have more than one colorant-containing layer, and thedifferent layers can have the same or different compositions, as long asthey all function as described above. The total thickness of thecombined colorant-containing layers is usually in the range given above.

Additional Additives

Other materials can be present as additives in the colorant-containinglayer as long as they do not interfere with the essential function ofthe layer. Examples of such additives include stabilizers, coating aids,plasticizers, flow additives, slip agents, antihalation agents,antistatic agents, surfactants, and others which are known to be used inthe formulation of coatings. However, it is typical to minimize theamount of additional materials in this layer, as they may deleteriouslyaffect the final product after transfer. Additives may add unwantedcolor for color proofing applications, or they may decrease durabilityand print life in lithographic printing applications.

Additional Layers:

The thermally imageable element may have additional layers. For example,an antihalation layer may be used on the side of the flexible ejectionlayer opposite the colorant-containing layer. Materials which can beused as antihalation agents are well known in the art. Other anchoringor subbing layers can be present on either side of the flexible ejectionlayer and are also well known in the art.

In some embodiments of this invention, a material functioning as a heatabsorber and a colorant is present in a single layer, termed the toplayer. Thus the top layer has a dual function of being both a heatinglayer and a colorant-containing layer. The characteristics of the toplayer are the same as those given for the colorant-containing layer. Atypical material functioning as a heat absorber and colorant is carbonblack.

An overcoat layer may also be present above the thermally imageablecolorant-containing layer. The bleach agent for bleaching the lightattenuating agent which may be present in the thermally imageablecolorant-containing layer may be present in the overcoat layer. If thelight attenuating agent is present in the overcoat layer, the lightattenuating agent will have to be brought into contact with the overcoatlayer prior to formation of the final element from a different source.

Yet additional thermally imageable elements may comprise alternatecolorant-containing layer or layers on a support. Additional layers maybe present depending of the specific process used for imagewise exposureand transfer of the formed images. Some suitable thermally imageableelements are disclosed in U.S. Pat. Nos. 5,773,188, 5,622,795,5,593,808, 5,156,938, 5,256,506, 5,171,650 and 5,681,681.

Receiver Element

The receiver element (20), shown in FIG. 2, is the part of the laserableassemblage, to which the exposed areas of the thermally imageable layer,typically comprising a polymeric binder and a pigment, are transferred.In most cases, the exposed areas of the thermally imageable layer willnot be removed from the thermally imageable element in the absence of areceiver element. That is, exposure of the thermally imageable elementalone to laser radiation does not cause material to be removed, ortransferred. The exposed areas of the thermally imageable layer, areremoved from the thermally imageable element only when it is exposed tolaser radiation and the thermally imageable element is in contact withor adjacent to the receiver element. In one embodiment, the thermallyimageable element actually touches the surface of the image receivinglayer of the receiver element.

The receiver element (20) may be non-photosensitive or photosensitive.

The non-photosensitive receiver element usually comprises a receiversupport (21) and an image receiving layer (22). The receiver support(21) comprises a dimensionally stable sheet material. The assemblage canbe imaged through the receiver support if that support is transparent.Examples of transparent films for receiver supports include, for examplepolyethylene terephthalate, polyether sulfone, a polyimide, a poly(vinylalcohol-co-acetal), polyethylene, or a cellulose ester, such ascellulose acetate. Examples of opaque support materials include, forexample, polyethylene terephthalate filled with a white pigment such astitanium dioxide, ivory paper, or synthetic paper, such as Tyvek®spunbonded polyolefin made by E.I. du Pont de Nemours and Company ofWilmington, Del. Paper supports are typical for proofing applications,while a polyester support, such as poly(ethylene terephthalate) istypical for a medical hardcopy and color filter array applications.Roughened supports may also be used in the receiver element.

The image receiving layer (22) may comprise one or more layers whereinoptionally the outermost layer is comprised of a material capable ofbeing micro-roughened. Some examples of materials that are usefulinclude a polycarbonate; a polyurethane; a polyester; polyvinylchloride; styrene/acrylonitrile copolymer; poly(caprolactone);poly(vinylacetate), vinylacetate copolymers with ethylene and/or vinylchloride; (meth)acrylate homopolymers (such as butyl-methacrylate) andcopolymers; and mixtures thereof. Typically the outermost imagereceiving layer is a crystalline polymer or poly(vinylacetate) layer.The crystalline image receiving layer polymers, for example,polycaprolactone polymers, typically have melting points in the range ofabout 50 to about 64° C., more typically about 56 to about 64° C., andmost typically about 58 to about 62° C. Blends made from 5-40% Capa® 650(melt range 58-60° C.) and Tone® P-300 (melt range 58-62° C.), bothpolycaprolactones, are particularly useful as the outermost layer inthis invention. Typically, 100% of CAPA 650 or Tone P-300 is used.However, thermoplastic polymers, such as polyvinyl acetate, have highermelting points (softening point ranges of about 100 to about 180° C.).Useful receiver elements are also disclosed in U.S. Pat. No. 5,534,387wherein an outermost layer optionally capable of being micro-roughened,for example, a polycaprolactone or poly(vinylacetate) layer is presenton the ethylene/vinyl acetate copolymer layer disclosed therein. Theethylene/vinyl acetate copolymer layer thickness can range from about0.5 to about 5 mils and the polycaprolactone layer thickness from about2 to about 100 mg/dm². Typically, the ethylene/vinyl acetate copolymercomprising more ethylene than vinyl acetate.

One preferred example is the WaterProof® Transfer Sheet sold by DuPontunder Stock #G06086 having coated thereon a polycaprolactone orpoly(vinylacetate) layer. This image receiving layer can be present inany amount effective for the intended purpose. In general, good resultshave been obtained at coating weights in the range of about 5 to about150 mg/dm², typically about 20 to about 60 mg/dm².

The image receiving layer or layers described above may optionallyinclude one or more other layers between the receiver support and theimage receiving layer. A useful additional layer between the imagereceiving layer and the support is a release layer. The receiver supportalone or the combination of receiver support and release layer isreferred to as a first temporary carrier. The release layer can providethe desired adhesion balance to the receiver support so that theimage-receiving layer adheres to the receiver support during exposureand separation from the thermally imageable element, but promotes theseparation of the image receiving layer from the receiver support insubsequent steps. Examples of materials suitable for use as the releaselayer include polyamides, silicones, vinyl chloride polymers andcopolymers, vinyl acetate polymers and copolymers and plasticizedpolyvinyl alcohols. The release layer can have a thickness in the rangeof about 1 to about 50 microns.

A cushion layer which is a deformable layer may also be present in thereceiver element, typically between the release layer and the receiversupport. The cushion layer may be present to increase the contactbetween the receiver element and the thermally imageable element whenassembled. Additionally, the cushion layer aids in the optionalmicro-roughening process by providing a deformable base under pressureand optional heat. Furthermore, the cushion layer provides excellentlamination properties in the final image transfer to a paper or othersubstrate. Examples of suitable materials for use as the cushion layerinclude copolymers of styrene and olefin monomers; such as,styrene/ethylene/butylene/styrene, styrene/butylene/styrene blockcopolymers, ethylene-vinylacetate and other elastomers useful as bindersin flexographic plate applications. The cushion layer may have athickness range from about 0.5 to about 5 mils (or higher).

Methods for optionally roughening the surface of the image receivinglayer include micro-roughening. Micro-roughening may be accomplished byany suitable method. One specific example, is by bringing it in contactwith a roughened sheet typically under pressure and heat. The pressuresused may range from about 800 +/−about 400 psi. Optionally, heat may beapplied up to about 80 to about 88° C. (175 to 190° F.) more typicallyabout 54.4° C. (130° F.) for polycaprolactone polymers and about 94° C.(200° F.) for poly(vinylacetate) polymers, to obtain a uniformmicro-roughened surface across the image receiving layer. Alternatively,heated or chilled roughened rolls may be used to achieve themicro-roughening.

It is typical that the means used for micro-roughening of the imagereceiving layer has a uniform roughness across its surface. Typically,the means used for micro-roughening has an average roughness (Ra) ofabout 1μ and surface irregularities having a plurality of peaks, atleast about 20 of the peaks having a height of at least about 200 nm anda diameter of about 100 pixels over a surface area of about 458μ byabout 602μ.

The roughening means should impart to the surface of the image receivinglayer an average roughness (Ra) of less than about 1μ, typically lessthan about 0.95μ, and more typically less than about 0.5μ, and surfaceirregularities having a plurality of peaks, at least about 40 of thepeaks, typically at least about 50 of the peaks, and still moretypically at least about 60 of the peaks, having a height of at leastabout 200 nm and a diameter of about 100 pixels over a surface area ofabout 458μ by about 602μ. These measurements are made using WycoProfilometer (Wyko Model NT 3300) manufactured by Veeko Metrology,Tucson, Ariz.

The outermost surface of the receiver element may further comprise agloss reading of about 5 to about 35 gloss units, typically about 20 toabout 30 gloss units, at an 85° angle. A GARDCO 20/60/85 degreeNOVO-GLOSS meter manufactured by The Paul Gardner Company may be used totake measurements. The glossmeter should be placed in the sameorientation for all readings across the transverse directionorientation.

The topography of the surface of the image receiving layer may beimportant in obtaining a high quality final image with substantially nomicro-dropouts.

The receiver element is typically an intermediate element in the processof the invention because the laser imaging step is normally followed byone or more transfer steps by which the exposed areas of the thermallyimageable layer are transferred to the permanent substrate.

Permanent Substrate

One advantage of the process of this invention is that the permanentsubstrate for receiving the colorant-containing image can be chosen fromalmost any sheet material desired. For most proofing applications apaper substrate is used, typically the same paper on which the imagewill ultimately be printed. An example of a paper substrate is LOEpaper. However, almost any paper stock can be used. Other materialswhich can be used as the permanent substrate include cloth, wood, glass,china, most polymeric films, synthetic papers, thin metal sheets orfoils, etc. Almost any material which will adhere to the thermoplasticpolymer layer, can be used as the permanent substrate.

Autofocus Process Steps

The process for adjusting the energy of an imaging laser for imaging athermally imageable element comprises the steps of:

(a) providing an imaging unit having a non-imaging laser and an imaginglaser, the non-imaging laser having a light detector which is incommunication with the imaging laser;

(b) contacting a receiver element with the thermally imageable elementin the imaging unit, the thermally imageable element comprising athermally imageable layer on a front side of a base element and a lightattenuated layer on a back side of the base element comprising a lightattenuating agent;

(c) actuating the non-imaging laser to expose the thermally imageableelement and the receiver element to an amount of light energy sufficientfor the light detector to detect the amount of light reflected from thelight attenuated layer of the thermally imageable element and thereceiver element, whereby light reflected from interfaces beyond theback surface of the light attenuated layer is substantially reduced andis substantially dominated by the light reflecting from the lightattenuated layer of the thermally imageable element into the lightdetector; and

(d) actuating the imaging laser to properly focus the imaging laser inorder to expose the thermally imageable element to an amount of lightenergy sufficient for imaging the thermally imageable element, the focusof light energy being determined by the amount of light reflected fromthe light attenuated layer of the thermally imageable element andcommunicated to the imaging laser by the light detector.

The imaging unit has a non-imaging laser and an imaging laser, thenon-imaging laser having a light detector which is in communication withthe imaging laser. Typically the non-imaging laser emits in about the300 nm to about the 1500 nm region. The non-imaging laser is not used toimage the thermally imageable element, and is therefore constantlyoperational prior to and during imaging for focussing the imaging laserthereby adjusting the energy to the imaging laser for the imaging step.In one embodiment, the non-imaging laser may emit in the 670 nm regionand the imaging laser may emit in about the 750 to 850 nm region. Anexample of a non-imaging laser is the Toshiba (Japan) 10 mW, 670 nmvisible light laser diode. Suitable imaging lasers may be obtained fromSpectra Diode Laboratries, San Jose, Calif. or Sanyo Electric Co.,Osaka, JP. These may be used as part of a laser-spatial light modulatorsystem such as that disclosed in U.S. Pat. No. 5,517,359, orelectrically modulated directly as disclosed in U.S. Pat. No. 4,743,091.Some typically used light detectors, also known as position sensitivedetectors include monolithic Silicon detectors comprising 2, 4, or asimilar number of elements arrayed such that the portion of reflectedbeam on each segment can be measured, and the relative position of afeature such as the center of the beam can be determined. Suitable lightdetectors may be obtained from United Detector Technology (U.S.A.).Alternately, the position of the beam could be determined from a sensorhaving greater than 4 elements, such as a CCD or CMOS sensor having 1024to 10,000,000 elements, as used in television image inspection systems.An example is the KAF-0400 from Eastman Kodak Co., Rochester, N.Y. Oneexample of an imaging unit is that disclosed in U.S. Pat. No. 6,137,580.

As shown in FIGS. 3 and 4, the optional carrier element (71), thereceiver element (20) having the light attenuated layer, and thethermally imageable element (10) are positioned over a drum (70) whichis part of an imaging unit. One example of an imaging unit is the CREOSpectrum Trendsetter which utilizes a loading cassette. The optionalcarrier element may have a series of holes along the edges of theelement as shown to assist in the drawing of a vacuum prior to theimaging step. The thermally imageable element (10), and the receiverelement (20) may be loaded into the cassette in this order with aninterleaving sheet present between each of the specified elements. Atleast one additional thermally imageable element (10), may also beloaded into the cassette.

In FIG. 5, after contact of the thermally imageable element and thereceiver element is achieved, the probe beam light (40) from thenon-imaging laser is emitted in the direction of the sandwich formed bythe optional carrier element (71), the receiver element (20) and thethermally imageable element (10).

As shown in FIG. 5, wherein the thermally imageable element does notcomprise a light attenuated layer, the light reflected off the backsurface of the thermally imageable element and seen by light detector(50) is depicted by (41), the light reflected off the receiver elementis depicted as (42), and the light reflected off the carrier element isdepicted as (43). Those skilled in the art will recognize that each ofthese reflections may be comprised of individual reflections produced ateach interface where the optical properties change, and each reflectionwill have wavelength dependant amplitude and phase. (51) representsmultiple reflected spots from the thermally imageable element (10), thereceiver element (20) and the optional carrier element (71) onto thelight detector (50).

The multiple reflected spots (51) could comprise 10 or more individualbeams for the optical sandwich depicted in FIG. 3. The light detector,typically a position sensitive detector and its associated electronicsand optional processing computer determines the position of the planeonto which to focus the imaging laser light based on these varyingsignals from the reflected light as the sandwich moves under the imagingsystem which includes the imaging laser. This determination of theoptimum focus position is then communicated to the imaging laser.

The focus position is the distance in microns that the imaging laserbeam travels into the thermally imageable element (color donorstructure). The distance is measured starting from the outermost surfaceof the thermally imageable element and ending at the point where thebeam reaches either the surface of the metal layer (if present) or thesurface of the thermally imageable layer which is closest to the laser.The distance is measured empirically by imaging equipment software. Thisdistance may not correspond exactly to the thicknesses of the layers ofthe thermally imageable element as measured by conventional means suchas, a micrometer, because the laser beam does not travel perpendicularto the thermally imageable element. There can be some variation in focuspositions for a given set of films as the imaging laser source ages andwhen films of the same color have different thicknesses because ofnon-uniformity of the thicknesses of the layers making up the thermallyimageable element. The imaging laser is then actuated to focus theimaging laser in order to expose the thermally imageable element to anamount of light energy sufficient for imaging the thermally imageableelement, the focus of light energy being determined by the amount oflight reflected from the light attenuated layer of the thermallyimageable element and the receiver element and communicated to theimaging laser by the light detector. Where one or more of the reflectednon-imaging beams is spurious or otherwise makes determination of theposition of the media sandwich erroneous or indeterminate, focusingerrors of the imaging beam can occur. Elimination or reduction ofreflected light from the interfaces beyond the light attenuated layerhave been found to improve the accuracy of determining the properfocusing position for the imaging laser.

Imaging Process Steps

Exposure:

The first step in the process of the invention is imagewise exposing thelaserable assemblage to laser radiation. The exposure step is typicallyeffected with an imaging laser at a laser fluence of about 600 mJ/cm² orless, most typically about 250 to about 440 mJ/cm². The laserableassemblage comprises the thermally imageable element and the receiverelement.

The assemblage is normally prepared following removal of acoversheet(s), if present, by placing the thermally imageable element incontact with the receiver element such that thermally imageable layeractually touches the image receiving layer on the receiver element.Vacuum and/or pressure can be used to hold the two elements together. Asone alternative, the thermally imageable and receiver elements can beheld together by fusion of layers at the periphery. As anotheralternative, the thermally imageable and receiver elements can be tapedtogether and taped to the imaging apparatus, or a pin/clamping systemcan be used. As yet another alternative, the thermally imageable elementcan be laminated to the receiver element to afford a laserableassemblage. The laserable assemblage can be conveniently mounted on adrum to facilitate laser imaging. Those skilled in the art willrecognize that other engine architectures such as flatbed, internaldrum, capstan drive, etc. can also be used with this invention.

Various types of lasers can be used to expose the laserable assemblage.The laser is typically one emitting in the infrared, near-infrared orvisible region. Particularly advantageous are diode lasers emitting inthe region of about 750 to about 870 nm which offer a substantialadvantage in terms of their small size, low cost, stability,reliability, ruggedness and ease of modulation. Diode lasers emitting inthe range of about 780 to about 850 nm are most typical. Such lasers areavailable from, for example, Spectra Diode Laboratories (San Jose,Calif.). One preferred device used for applying an image to the imagereceiving layer is the Creo Spectrum Trendsetter 3244F, which utilizeslasers emitting near 830 nm. This device utilizes a Spatial LightModulator to split and modulate the 5-50 Watt output from the ˜830 nmlaser diode array. Associated optics focus this light onto the imageableelements. This produces 0.1 to 30 Watts of imaging light on the donorelement, focused to an array of 50 to 240 individual beams, each with10-200 mW of light in approximately 10×10 to 2×10 micron spots. Similarexposure can be obtained with individual lasers per spot, such asdisclosed in U.S. Pat. No. 4,743,091. In this case each laser emits50-300 mW of electrically modulated light at 780-870 nm. Other optionsinclude fibre coupled lasers emitting 500-3000 mW and each individuallymodulated and focused on the media. Such a laser can be obtained fromOpto Power in Tucson, Ariz.

Optical imaging systems can be constructed based on any of these laseroptions. In each system, focus of the imaging laser can be determinedmanually or automatically. A common autofocus approach utilizes aseparate non-imaging laser incident on the desired imaging plane andreflected into a sensor. There are many approaches to the design of thisautofocus system, but they can be incorporated into imaging systemsbased on any exposure laser source.

The exposure may take place through the optional ejection layer orsubbing layer and/or the heating layer of the thermally imageableelement. The optional ejection layer or subbing layer or the receiverelement having a roughened surface, must be substantially transparent tothe laser radiation. The heating layer absorbs the laser radiation andassists in the transfer of the image forming material. In some cases,the ejection layer or subbing layer of the thermally imageable elementwill be a film that is transparent to infrared radiation and theexposure is conveniently carried out through the ejection or subbinglayer. In other cases, these layers may contain laser absorbing dyeswhich aid in material transfer to the image receiving element.

The laserable assemblage is exposed imagewise so that the exposed areasof the thermally imageable layer are transferred to the receiver elementin a pattern. The pattern itself can be, for example, in the form ofdots or line work generated by a computer, in a form obtained byscanning artwork to be copied, in the form of a digitized image takenfrom original artwork, or a combination of any of these forms which canbe electronically combined on a computer prior to laser exposure. Thelaser beam and the laserable assemblage are in constant motion withrespect to each other, such that each minute area of the assemblage,i.e., “pixel” is individually addressed by the laser. This is generallyaccomplished by mounting the laserable assemblage on a rotatable drum. Aflat bed recorder can also be used.

Separation:

The next step in the process of the invention is separating thethermally imageable element from the receiver element. Usually this isdone by simply peeling the two elements apart. This generally requiresvery little peel force, and is accomplished by simply separating thethermally imageable support from the receiver element. This can be doneusing any conventional separation technique and can be manual orautomatic without operator intervention.

Separation results in a laser generated image, typically a halftone dotimage, comprising the transferred exposed areas of the thermallyimageable layer, being revealed on the image receiving layer of thereceiver element. Typically the image formed by the exposure andseparation steps is a laser generated halftone dot color image formed ona crystalline polymer layer, the crystalline polymer layer being locatedon a first temporary carrier which may or may not have a layer presentdirectly on it prior to application of the crystalline polymer layer,wherein either the first temporary carrier or the optional layer thatmay be present directly on it comprise the light attenuating agent.

Additional Steps:

The so revealed image on the image receiving layer may then betransferred directly to a permanent substrate or it may be transferredto an intermediate element such as an image rigidification element, andthen to a permanent substrate. Typically, the image rigidificationelement comprises a support having a release surface and a thermoplasticpolymer layer.

The so revealed image on the image receiving layer is then brought intocontact with, typically laminated to, the thermoplastic polymer layer ofthe image rigidification element resulting in the thermoplastic polymerlayer of the rigidification element and the image receiving layer of thereceiver element encasing the image. A WaterProof® Laminator,manufactured by DuPont is preferably used to accomplish the lamination.However, other conventional means may be used to accomplish contact ofthe image carrying receiver element with the thermoplastic polymer layerof the rigidification element. It is important that the adhesion of therigidfication element support having a release surface to thethermoplastic polymer layer be less than the adhesion between any otherlayers in the sandwich. The novel assemblage or sandwich is highlyuseful, e.g., as an improved image proofing system. The support having arelease surface may then removed, typically by peeling off, to revealthe thermoplastic film. The image on the receiver element may then betransferred to the permanent substrate by contacting the permanentsubstrate with, typically laminating it to, the revealed thermoplasticpolymer layer of the sandwich. Again a WaterProof® Laminator,manufactured by DuPont, is typically used to accomplish the lamination.However, other conventional means may be used to accomplish thiscontact.

Another embodiment includes the additional step of removing, typicallyby peeling off, the receiver support resulting in the assemblage orsandwich comprising the permanent substrate, the thermoplastic layer,the image, and the image receiving layer. In a more typical embodiment,these assemblages represent a printing proof comprising a lasergenerated halftone dot color thermal image formed on a crystallinepolymer layer, and a thermoplastic polymer layer laminated on onesurface to said crystalline polymer layer and laminated on the othersurface to the permanent substrate, whereby the image is encased betweenthe crystalline polymer layer and the thermoplastic polymer layer.

Formation of Multicolor Images:

In proofing applications, the receiver element can be an intermediateelement onto which a multicolor image is built up. A thermally imageableelement having a thermally imageable layer comprising a first pigment isexposed and separated as described above. The receiver element has animage formed with the first pigment, which is typically a lasergenerated halftone dot color thermal image. Thereafter, a secondthermally imageable element having a thermally imageable layer differentfrom that of the first thermally imageable element forms a laserableassemblage with the receiver element having the image of the firstpigment and is imagewise exposed and separated as described above. Thesteps of (a) forming the laserable assemblage with a thermally imageableelement having a different pigment than that used before and thepreviously imaged receiver element, (b) exposing, and (c) separating aresequentially repeated as often as necessary in order to build themulti-colorant-containing image of a color proof on the receiverelement. The image on the receiver therefore changes as the image isbuilt up, and the transmission of this image at the wavelength of thenon-imaging laser changes as the process is repeated. Light passingthrough this image and reflected into the light detector, typically aposition sensitive light detector, causes imaging errors, which aregreatly reduced by the light attenuated layer in the receiver.

The rigidification element may then be brought into contact with,typically laminated to, the multiple colorant-containing images on theimage receiving element with the last colorant-containing image incontact with the thermoplastic polymer layer. The process is thencompleted as described above.

EXAMPLES

These non-limiting examples demonstrate the processes and productsdescribed herein wherein images of a wide variety of colors areobtained. All percentages are weight percentages unless indicatedotherwise.

Glossary SDA 2-[2-[2-Chloro-3[2-(1,3-dihydro-1,1dimethyl-3-(4dimethyl-3(4sulfobutyl)-2H-benz[e]indol-2-yllidene)ethylidene]-1-cyclohexen-1-yl]ethenyl]-1,1-dimethyl-3-(sulfobutyl)-1H-benz[e]indolium, inner salt, free acid SDA4927 Infrared dye [CAS No. 162411-28-1] (H. W. Sands Corp., Jupiter, FL)FSA Zonyl ® FSA fluoro surfactant; 25% solids in water and isopropanol,[CAS No. 57534-45-7] A lithium carboxylate anionic fluorosurfactanthaving the following structure: RfCH2CH2SCH2CH2CO2Li where Rf =F(CF2CF2)x and where x = 1 to 9 (DuPont, Wilmington, DE) FSD Zonyl ® FSDfluoro surfactant; 43% active ingredient in water (DuPont, Wilmington,DE) RCP-26735 Methylmethacrylate/n-butylmethacrylate (76/24) copolymerlatex emulsion at 37.4% solids (DuPont, Wilmington, DE). PEG 6800Polyethylene glycol 6800 [CAS No. 25322-68-3], 100%, Scientific PolymerProducts, Inc., Ontario, NY) DF110D Surfynol ® DF110D (Air Products)Zinpol ® 20 Zinpol ® 20, Polyethylene wax emulsion, 35% in water (B. F.Goodrich Company) Melinex ® 573 4 mil clear PET base(DuPontTeijinFilms ™, a joint venture of E. I. du Pont de Nemours &Company) Melinex ® 6442 4 mil PET base with 670 nm dye absorber(DuPontTeijinFilms ™, a joint venture of E. I. du Pont de Nemours &Company) Dye is CAS # 12217-80-0 1H-Naphth[2,3-f]isoindole-1,3,5,10(2H)-tetrone, 4,11-diamino-2-(3-methoxy- propyl)-(9Cl) (CA INDEXNAME) 30S330 Green Shade Phthalo Blue Waterborne Dispersion 40% solids(Penn Color, Inc., Doylestown, PA) 32Y144D Green Shade Yellow WaterborneDispersion 41% solids (Penn Color, Inc., Doylestown, PA) 32Y145D RedShade Yellow Waterborne Dispersion 40% solids (Penn Color, Inc.,Doylestown, PA) 32R164D Red 32R164D pigment dispersion; 40% in water(Penn color, PA) 32S168D Violet 32S168D pigment dispersion; 41% in water(Penn Color, PA) 32S187D Blue 32S187D pigment dispersion; 40% in water(Penn Color, PA) WaterProof ® Transfer Sheet Stock Number H74900 (akaReceiver) Thermal Halftone IRL Film Stock Number H71103 Proofing DonorFilm Black Stock Number H71073 System—4 Page Donor Film Magenta StockNumber H71022 size

Example 1 Preparation of the Thermally Imageable Compositions

This example shows the preparation of a 670 nm absorbing coatablecomposition and a thermally imageable element. The back side of amagenta colored thermally imageable element was coated with a 670 nmabsorbing composition and used in a color proofing application. Thethermally imageable element was made from a 4 mil polyester backing(Melinex® 573) sputtered with about 70 Å of chromium, sufficient toproduce about 60% transmission of light, by CP Films (Martinsville,Va.). The metal thickness was monitored in situ using a quartz crystaland after deposition by measuring reflection and transmission of thefilms. This metalized base was then coated with the Magenta donorformula described in Table 1 using production equipment. A compositionof a 670 nm absorbing pigment dispersion was prepared using the recipein Table 2, then coated on the back side of the magenta element (theside of the base element opposite the front side which was coated withthe magenta formula) using DuPont's WaterProof® Color Versatility coaterand wire rods #5, #6, and #7 followed by drying at 50° C. for 5 minutes.

TABLE 1 Recipes for colorant-containing compositions: Ingredient MagentaYellow Cyan Deionized Water 12,294 18,050 15,433 RCP 26735 4,326 4,1336,941 32R164D 1,526 32S168D 19.2 32Y144D 1,321 32Y145D 257.7 30S3301,259 32S187D 160.2 PEG 146.3 153.8 135.4 SDA 4927 53.2 48.1 50.7 DF110D12.2 FSA 26.6 24.2 19.5 TOTAL (grams) 19,000 24,000 24,000

TABLE 2 Recipe for 670 nm absorbing coating: Ingredient AbsorberDistilled Water 129.7 RCP 26735  59.4 30S330  9.5 PEG  1.1 FSD  0.3TOTAL (grams) 200.0

Each of the so prepared back-side coated magenta elements prepared asdescribed above were placed in the cassette of a Creo 3244 SpectrumTrendsetter, Creo, Vancouver, BC, and imaged onto a receiver todetermine their focus positions for hot focus: 120-200 rpm and 12-18wafts. The computer attached to the Trendsetter contained digital datafiles representing the 4 process colors (yellow, magenta, cyan andblack).

This imaging equipment produced laser generated magenta color thermaldigital halftone images (proofs).

The image was transferred to an image rigidification element (IRL). Thereceiver support was peeled off and the image was contacted with an LOEpaper substrate followed by peeling off of the image rigidificationelement support to form an image on the LOE paper substrate sandwichedbetween the polycaprolactone layer and the IRL polymer layer).

The results in Table 3 show that the difference between the focusposition for single and overprint images was less for the magentaelement with a 670 nm absorbing back side coating than for the controlelement without the backside coating containing a 670 nm absorber.

Focus position data used in these examples was collected from thecomputer diagnostic port of the Creo 3244 Spectrum Trendsetter.

TABLE 3 Focus Position of Magenta Element Focus Position CoatingAbsorbance Single Over- Wire Rod # weight mg/dm 670 nm Color Print 5 6.1.33 60 70 6 9.2 .40 65 80 7 12.5  .44 70 80 Control 30 60Control sample is a magenta element without the back side coating of a670 nm absorber.

1. A process for adjusting the focus of an imaging laser for imaging athermally imageable element comprises the steps of: (a) providing animaging unit having a non-imaging laser and an imaging laser, thenon-imaging laser having a light detector which is in communication withthe imaging laser; (b) contacting a receiver element with the thermallyimageable element in the imaging unit, the thermally imageable elementcomprising a thermally imageable layer on a front side of a base elementand a light attenuated layer on a back side of the base elementcomprising a light attenuating agent; (c) actuating the non-imaginglaser to expose the thermally imageable element and the receiver elementto an amount of light energy sufficient for the light detector to detectthe amount of light reflected from the light attenuated layer of thethermally imageable element and the receiver element; and (d) actuatingthe imaging laser to focus the imaging laser in order to expose thethermally imageable element to an amount of light energy sufficient forimaging the thermally imageable element, the focus of light energy beingdetermined by the amount of light reflected from the light attenuatedlayer of the thermally imageable element and communicated to the imaginglaser by the light detector.
 2. The process of claim 1 wherein the lightattenuating agent is selected from an absorber, a diffuser and mixturesthereof.
 3. The process of claim 2 wherein the absorber is a bluephthalocyanine pigment.
 4. The process of claim 2 wherein the absorberis carbon black.
 5. The process of claim 2 wherein the diffuser istitanium dioxide.
 6. The process of claim 1 wherein the lightattenuating agent is a mixture of a blue phthalocyanine pigment andtitanium dioxide.
 7. The process of claim 1 wherein the thermallyimageable layer comprises a pigment.
 8. The process of claim 1 whereinthe non-imaging laser emits light at a wavelength ranging from about 300nm to about 1500 nm.
 9. The process of claim 1 further comprising thesteps of: (a) imaging the thermally imageable element to form imaged andnon-imaged areas; and (b) separating the imaged thermally imageableelement from the receiver element to form an image on the receiverelement.
 10. A laser imageable element comprising a base element havinga front side and a back side, a thermally imageable layer being locatedon the front side of the base element and a light attenuated layer beinglocated on the back side of the base element, the light attenuated layercomprising a light attenuating agent, wherein a heating layer isdisposed between the base element and the thermally imageable layer.